Add missing NULLIF (#1625)

In some cases that empty aggregated brunnel attributes was coming through in the tiles in the transportation_name layer.

layers/transportation_name/transportation_name.sql

@@ -47,7 +47,7 @@ SELECT geometry,
                THEN highway
            ELSE subclass
            END AS subclass,
-       brunnel,
+       NULLIF(brunnel, '') AS brunnel,
        NULLIF(layer, 0) AS layer,
        "level",
        CASE WHEN indoor = TRUE THEN 1 END AS indoor